Conventionally, as a glow plug for assisting starting of a diesel engine, or the like, a glow plug using a sheathed heater is known. The sheathed heater includes a metal tube (sheath tube) with a closed front end in which a heating coil is housed while enclosing an insulating powder such as MgO powder. As the materials of the heating coil, a Fe—Cr—Al alloy, a Ni—Cr alloy, and the like are known.
The Fe—Cr—Al alloy has a high melting point of 1520° C. On the other hand, the Ni—Cr alloy has a melting point of 1370° C., which is 150° C. lower than the Fe—Cr—Al alloy. Thus, the use of Ni—Cr alloy as the material of the heating coil may cause erosion of the heating coil at rapidly increasing temperatures. Hence, the Fe—Cr—Al alloy is commonly used as the material of the heating coil in the art. Furthermore, a control coil including Ni or Fe as a principal component and connected in series with a heating coil of Fe—Cr—Al alloy has been known as a structure that enables a rapid increase in temperature of the heating coil while preventing an excessive increase in temperature thereof (see, for example, Patent Document 1).